Clarify docs for creating inline commit comments

Update docs in response to [Halp ticket][1].

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The `position` parameter identifies the position of the comment within the diff. Use the `position` parameter for this purpose, not the `line` parameter.

The `line` parameter is a vestige of an earlier version of the API.
[1]: https://halp.githubapp.com/search/jasonrudolph-api-new-open-589393/discussions/b9883128007a11e38581d07913e051b5